1. Field of the Invention
The invention concerns a flame monitoring system and a method of monitoring a flame.
2. Description of the Prior Art
For the purposes of monitoring oil, gas or coal dust flames, it is known to use flame monitoring systems and methods which utilise the fluctuations in intensity of the flame in the infra-red spectral range. The advantage of such systems is that they are suitable for all kinds of fuels and thus there is no need for fuel-specific monitoring modes in the case of multiple-fuel burners, for example involving the detection of UV-radiation in the case of gas and visible radiation in the case of heavy oil. Disadvantages with IR-monitoring however are that both the slow variations in intensity of a furnace wall involving afterglow phenomenaxe2x80x94so-called striation or schlieren frequenciesxe2x80x94and also the fast changes in light sources which are generally operated with mains ac voltage can simulate a flame. If artificial light happens to shine into the burner or combustion chamber during operation or also during the maintenance of burner systems, IR-monitoring would indicate the presence of a flame.
Filtration of striation or schlieren frequencies which are specified in various publications as being up to 3 Hz can be relatively easily implemented by means of high pass filters, in which case the flame frequencies produced by the combustion operation at above 10 Hz are not cut thereby. If however the harmonics of the mains frequency have to be suppressed by being filtered out, that makes matters more expensive and gives rise to more severe problems. This method necessarily also involves the loss of information from the flame, particularly if the mains frequency is subject to wide tolerances or if various rated frequency ranges have to be covered. The European equipment standard EN298 which is relevant for flame monitoring apparatuses also allows the option of shut-down of the flame sensor being effected by a suitable flame sensor fixing system if it is removed from the fixing. At any event the safeguard in relation to extraneous light is to be guaranteed even when consideration is given to primary and secondary faults, in accordance with EN298. With the last-mentioned method, it is thought that this is extremely difficult to achieve as proper operability, for example of a limit switch, can only be tested by actually removing the flame sensor from its fixing.
There is therefore a need to achieve immunity in relation to mains frequency-modulated extraneous light sources, by electronic means, either by circuits which are fail-safe in themselves or by cyclic testing, in the case of monitoring systems designed for continuous operation during operation of the burner.
EP 0 320 082 A1 describes a flame monitoring circuit in which just evaluation of the alternating light component of a flame is utilised as a measure for fail-safe flame detection. That structure however only affords a safeguard in relation to flame simulation as long as the safety-relevant ambient light referred to therein involves constant light. Light from generally ac voltage-operated extraneous light sources in contrast very definitely results in simulation of a flame and thus unsafe burner operation. In addition there is the danger that an internal component fault in the IC maintains actuation of the fuel valve in spite of the absence of a flame. For that reason alone use on burners in a continuous mode of operation is out of the question.
EP 0 334 027 A1 discloses a construction which is suitable in this respect, but the level of expenditure is disproportionately high as a result of the completely two-channel nature, and immunity in relation to mains-frequency alternating light signals is achieved with frequency-selective arrangements, the disadvantage of which, in terms of loss of flame signal information, has already been mentioned.
One way of obviating that deficiency is disclosed in EP 0 229 265 A1. In that case, mains frequency-harmonic signals are blocked with a high level of selectivity so that the information loss from the flame signal is kept very slight. The applicability to burners in a continuous mode of operation is doubtful however because an internal component fault, for example in the flip-flop, with flame simulation as a consequence, is not detected in operation, and immunity in relation to mains-frequency alternating light signals could at best be established in the burner stoppage condition.
The object of the present invention is to provide a flame monitoring system and a method of monitoring a flame, which has immunity in relation to mains frequency-harmonic input signals with a very low level of flame signal information loss and which is suitable for use in relation to burners in a continuous mode of operation.
In accordance with a first aspect of the invention, there is provided a flame monitoring system comprising:
a flame sensor which converts the radiation emanating from a flame into a flame signal;
a flame signal amplifier which converts the flame signal into an output signal; and
a frequency-selective arrangement which detects the presence of mains frequency-harmonic signals in the flame signal;
wherein:
the frequency-selective arrangement activates the flame signal amplifier when there are no mains frequency-harmonic signals in the flame signal; and
the frequency-selective arrangement deactivates the flame signal amplifier when there is a flame signal with mains frequency-harmonic signals or no flame signal or a test signal.
In accordance with a second aspect of the invention, there is provided a method of monitoring a flame, comprising:
converting radiation emanating from the flame into a flame signal which is converted into an output signal; and
detecting the presence of mains frequency-harmonic signals in the flame signal by using a frequency-selective arrangement;
wherein:
the flame signal is converted into an output signal when there are no mains frequency-harmonic signals in the flame signal; and
the flame signal is converted into a zero signal where there is a flame signal with mains frequency-harmonic signals or no flame signal or a test signal.
The present invention attains the stated object in that a flame sensor firstly converts the radiation issuing from a flame into a flame signal which in turn is transformed into an output signal by a flame signal booster or amplifier. A frequency-selective arrangement which is arranged in parallel with the flame signal amplifier also receives the flame signal itself and checks it for the presence of period signals. If the absence of mains frequency-harmonic signals is detected by the frequency-selective arrangement the, flame signal amplifier is activated while upon the detection of mains frequency-harmonic signals or in absence of a flame signal, the flame signal amplifier is deactivated. There is also the possibility of overwriting the flame signal with a test signal so that the input of the flame signal amplifier and also the input of the frequency selective arrangement can be acted upon by the test signal itself so that failures within the flame monitoring circuit, for example the failure of individual components, can be detected.
In that respect, the frequency-selective arrangement has a frequency detector which detects the presence of non-periodic flame signals and suitably activates or deactivates the flame signal amplifier by way of suitable switching means. That can be embodied in various ways.
On the one hand it is possible for the flame signal firstly to be boosted and converted into a rectangular signal, in which respect any reference signal can be used for that conversion operation. That rectangular signal then serves as a control signal of a bipolar current or voltage source which in turn feeds an integrator so that the output signal of the integrator fluctuates about a constant mean value with periodic input signals from the frequency detector. In other words, the bipolar current or voltage source charges and discharges the integrator depending on the fluctuation width of the input or the flame signal so that the averaged integration value is approximately zero in the case of periodic input signals.
The frequency-selective arrangement also has a coupler or a switch which firstly establishes whether the output signal of the frequency detector, that is to say the integrated input signal, remains within a defined switching threshold about a given mean value in order then to actuate a switch which suitably activates or deactivates the flame signal booster. If the frequency detector establishes that there is a purely periodic signal, the above-indicated switching threshold ensures that residual fluctuations in the integrated signal around the constant mean value or slight deviations around the zero value remain disregarded, which, depending on the respective limit frequency of the integrator, can also be caused by purely mains frequency-harmonic input signals.
Another option is that the frequency detector integrates the input signal, that is to say for example the flame signal, over previously fixedly defined periods, and the frequency-selective arrangement uses the integrated output signal for the actuation of a switch which in turn activates or deactivates the flame signal amplifier. By virtue of integration over those defined periods, it is possible to effect very tight, that is to say narrow-band filtering of discrete frequencies which are usually multiples of the mains frequencies so that here extraneous light components which follow the ac voltage of the mains frequency are sharply filtered out so that all other frequencies, that is to say in particular flame signals, can be detected in a virtually loss-free manner. In that respect it is appropriate for the frequency detector to be reset into its initial condition after each integration operation over one of the defined periods, otherwise drifting-away of the integrator output voltage could result in flame simulation, which in the test would be recognised as a component fault.
In that respect the flame signal is overwritable with a mains frequency-harmonic test signal so that the frequency detector then evaluates the test signal which permits checking of the circuit as such and detects the failure of individual components.
The frequency detector activates the switch in such a way that, in the case of the absence of a mains frequency-harmonic flame signal, the flame signal amplifier supplies a valid output signal while, upon the detection of mains frequency-harmonic input signals, at the frequency detector, the flame signal amplifier is deactivated so that a valid signal is not delivered at the output of the flame signal amplifier.
The mains frequency-harmonic test signal is advantageously applied at regular intervals of time in order always to have certainty about satisfactory functioning of the flame monitoring circuit.